A cataract is an occlusion of tissue comprising the lens in the eye. The lens, enclosed in a capsule, is situated immediately behind the pupil (FIG. 1), in front of the vitreous body, and encircled by the ciliary processes, which slightly overlap its margin.
The capsule of the lens is a transparent, highly elastic and brittle membrane, which closely surrounds the lens. It rests, behind, in the fossa petellaris in the fore part of the vitreous body; in front, it is in contact with the free boarder of the iris, this latter receding from it at the circumference, thus forming the posterior chamber of the eye; and it is retained in position chiefly by the suspensory ligament or zonules of the lens.
The lens is a transparent, bi-convex body, the convexity being greater in the posterior side than on the anterior side. The central points of its anterior and posterior surfaces are known as its anterior and posterior poles. It measures approximately 9 to 10 millimeters in the transverse diameter, and about 4 millimeters in the anteroposterior. It consists of concentric layers, of which the exterior are soft and easily detached (substantia corticalis); those beneath are firmer, the central ones forming a hardened nucleus (nucleus lentis). It is the clouding or opacity of these concentric layers which form the cataract.
In a cataract operation, the lens is removed from the eye (lens extraction). There are two principal types of lens extraction for cataracts in older patients--intracapsular and extracapsular. Intracapsular cataract extraction consists of removing the lens in its entirety through a 140.degree.-160.degree. superior limbal incision. In performing the intracapsular procedure, the time honored method has been to make a large limbal incision superiorly, grasp the lens with a metal capsule forceps and remove the lens intracapsularly. Later, a cryoprobe became a popular replacement for the metal forceps. In extracapsular cataract extractions, a superior limbal incision is also made, the anterior portion of the capsule is ruptured and removed, the nucleus is extracted, and the lens cortex is either irrigated or aspirated from the eye, leaving the posterior capsule behind.
In performing extracapsular cataract extractions, phacoemulsification and aspiration and phacofragmentation and irrigation procedures are commonly used. Using a small limbal incision, ultrasonic energy is used to break up the lens nucleus and cortex through an opening or hole in the anterior surface of the capsule. The rupturing of the anterior surface or wall of the capsule is made by piercing the wall with a cystitome, essentially a bent needle. In a procedure called "continuous circular capsulorehexis," the cystitome is used to make a small incision sufficient for forceps to grasp one side of the incision. Using forceps the anterior surface tissue of the capsule is torn in a circular fashion to produce a circular opening. In standard extracapsular cataract extractions not using phacoemulsification, the anterior capsule is removed, the corneo-scleral wound is opened to its full extent (8-10 mm) and the lens is removed from the eye. This removal can be done using a lens loop which scoops the lens and pulls it out of the capsule. Also, pressure (using a pair of forceps) can be applied to the surface of the eye inferiorly. This raises intravitreal pressure which delivers the lens through the wound opening.
In another technique, the cystitome is used to make perforations in a circular pattern about the anterior surface of the capsule and to the inside of the iris. Forceps are then used to tear the capsule tissue along the perforation.
Intracapsular cataract extraction, the old standard procedure for senile cataracts, presents the possibility of several serious post-surgical complications such as glacoma, retinal detachment, vitreous hemorrhage, infection, or epithelial downgrowth into the anterior chamber that prevents significant visual improvement.
A major disadvantage associated with each of the above capsulectomy techniques is that the direction of the tear is difficult to control. Structural weaknesses or imperfections in the capsule may cause the tear to be extremely irregular, even to the point where vitreous loss may occur. Use of the cystitome to perforate the capsule may initiate one or more stress tears, which when subjected to the tension of the tear, may deviate to a point where the integrity of the capsule jeopardizes the placement of an intraocular lens prothesis. At the very least, the capsulotomy tear produces an irregular boundary, making it difficult and time consuming to ascertain whether the intraocular lens prothesis has been properly positioned within the capsule.